In positron emission tomography (PET) imaging, a quantitative reconstruction of a tracer distribution requires attenuation correction. An attenuation map may be needed for attenuation correction. The attenuation map is typically acquired through a transmission scan using a computed tomography (CT) scanner or derived from magnetic resonance (MR) imaging. An attenuation map derived from MR is problematic in terms of accuracy. To provide an attenuation map from CT, a measured object may need to be exposed to a radiation dose. In another way, an attenuation map be acquired based on PET data and time-of-flight (TOF) information, but the quality and/or spatial resolution of the attenuation map is relatively poor. Therefore, it would be desirable to provide effective mechanisms for generating an attenuation map to correct activity distribution.